1. Field of the Invention
The present invention relates to a mobile communication system, and in particular, to a receiver, a mobile terminal, a base station, and a synchronization control method for use therewith.
2. Description of the Related Art
A code division multiple access (CDMA) scheme is a scheme in which a plurality of communication channels are multiplexed and are used for multiple accesses using a spectrum spreading code and hence has attracted attentions as a scheme to remarkably improve efficiency of frequency utilization. In a system using the CDMA, a transmission side multiplexes signals on channels necessary for communication such as a perch channel, a common control channel, a dedicated control channel, and a dedicated traffic channel by conducting a spectrum spreading modulation respectively with different spreading codes for the channels and transmits spread signals. On receiving the spread signals, a reception side despreads the signals with the spreading code used on the transmission side to demodulate a signal on a desired channel among the multiplexed signals. The signals modulated in the spectrum spreading modulation on the transmission side can be demodulated on the reception side only when both sides use the same spreading code and the same code phase. Therefore, an operation to establish synchronization and an operation to conduct tracking of synchronization on the reception side are regarded as essentially important techniques to implement the CDMA system. In the description, xe2x80x9csynchronization establishing operationxe2x80x9d indicates detection of a code phase of a spreading code used on the transmission side and xe2x80x9ctracking of synchronizationxe2x80x9d means keeping a code phase established.
In a phase to establish synchronization, a mobile terminal conducts a search for perch channels which the base stations around the terminal transmit. The perch channel includes information of a spreading code used by the base station and information necessary for power control to transmit them to the mobile terminal. The perch channel is almost continuously used by all base stations to send signals. A concrete method of establishing synchronization has been described in pages 57 to 63 of xe2x80x9cFast Cell Search Algorithm using Long Code Masking in DS-CDMA Asynchronous Cellular Systemxe2x80x9d written by Kenichi Higuchi, Kyoei Sawa, and Bunkoh Adachi in IEICE Technical Report RCS96-122, 1997-01.
The reason why the synchronization establishing and the tracking of synchronization are regarded as important is difficulty in the implementation of these operations as follows. In an actual propagation route, a propagation delay occurs in proportion to length of the route. Therefore, the code phase of the spreading code used on the reception side must be set to a value obtained by adding a phase shift associated with the propagation delay to the code phase on the transmission side. Furthermore, since the radio wave reflects at many places to arrive at the reception side through various paths, there appear a plurality of delayed waves. This phenomenon is generally called xe2x80x9cmultipathxe2x80x9d. Since the multipath brings about a plurality of received signal components having different phases on the reception side, the operation to establish synchronization and the synchronization tracking become difficult.
In the CDMA communication system, a rake receiving technique is employed to cope with the multipath. In the rake receiving technique, the delayed waves are separately demodulated with code phases corresponding to respective delay time of the waves, and resultant signals are combined with each other to obtain the diversity effect and a combined gain. To conduct the rake reception, it is required to simultaneously despread the received signal components having different phases. A technique of the prior art to implement the operation above is an invention of a matched filter described in JP-A-9-321664 entitled xe2x80x9cSpread Spectrum, Rake Receiver Using Time Window Control Loopxe2x80x9d. In the invention, despreading of a plurality of signals is simultaneously carried out with a plurality of spreading codes which are shifted by a fixed phase interval. In the matched filter, the despreading is accomplished in a predetermined phase range beginning at a code phase beforehand set. A time width corresponding to the predetermined phase range will be referred to as xe2x80x9ctime windowxe2x80x9d of the matched filter herebelow.
FIG. 14 shows a configuration diagram of a spread spectrum receiver of the related art. According to the related art, for a signal received by a radio frequency (RF) receiver 902, despread results corresponding to a plurality of code phases detected by matched filters 903 and 904 are inputted to an envelope detector 906 and a differential detector 905. For an output from the envelope detector 906, a maximum ratio combining controller 907 determines intensity of the despread results. According to the intensity of reception, signals detected by the differential detector 905 are combined with each other in a rake combining unit 910 to thereby achieve the rake reception. On the other hand, according to a result outputted from the envelope detector 906, a time window controller 908 and a sync tracking unit 909 control a time window location of the matched filters 903 and 904 such that a maximum peak position of an envelope aligns at a time window center location of the matched filters 903 and 904. Using an output from rake combining unit 910, a data reproducing unit 911 reproduces data.
In the related art, when there exist a plurality of delayed waves and there appear a plurality of correlation values corresponding to the respective waves, synchronization is aligned to one of the delayed waves which has a largest correlation value. Therefore, it can be expected in the related art that the tracking of synchronization is appropriately conducted only when a delayed wave having dominant intensity exists in a stable state among the delayed waves. However, in an actual environment, for example, in an urban environment, there exist many delayed waves and the respective delayed waves are not clearly different from each other in intensity in many cases. Moreover, fading is caused by movement of the mobile terminal and the respective delay waves strongly change in intensity depending on cases. As a result, the delayed wave having the largest correlation value frequently changes. Consequently, it is difficult in the related art to specify an appropriate code phase to be used for the tracking of synchronization.
In such an environment, received power is dispersed to the respective delayed waves and hence power per delayed wave is lowered. When either one of the delayed waves (paths) is selected for the tracking of synchronization, the overall received power can be only partly used for the tracking of synchronization and hence satisfactory stability cannot obtained.
It is therefore an object of the present invention to provide a synchronization control method, a receiver, a base station, and a mobile terminal which are particularly used in a CDMA mobile communication system and in which a state of out of synchronism is minimized and hence radio waves can be received in a stable state.
Another object of the present invention is that the tracking of synchronization is conducted in a stable state even in an environment such as an urban environment including a multipath and resultantly reduces the probability of call disconnection during a speech.
Further another object of the present invention is that a high-speed acquisition is conducted in an initial acquisition of synchronization and jitter due to noise is kept minimized after completion of the acquisition to stabilize the tracking of synchronization.
Still another object of the present invention is to minimize a period of time from when a terminal is powered to when the terminal is available for a speech and to reduce deterioration of speech quality due to phase jitter.
To solve the problem above in accordance with the present invention, the time window location of the matched filter is not aligned with a single path. Namely, a control operation is accomplished such that the time window location of the matched filter is aligned with a representative value which represents code phases corresponding to a plurality of paths for which valid correlational values have been detected and which represents spreading code phases of a plurality of paths obtained according to the respective correlational values. As the representative value, it is favorable to employ, for example, a center of path power.
In accordance with a first aspect of the present invention, there is provided a synchronization control method, comprising the steps of obtaining, according to a correlation between a received spread spectrum signal and an inputted spreading code, a delay profile including as elements thereof respective correlation values of a plurality of code phases, conducting a rake combining of despread signals according to the obtained delay profile and producing a decoded signal, calculating a representative value representing spreading code phases of a plurality of paths according to a plurality of correlation values and code phases thereof obtained from the delay profile obtained, and conducting a control operation for a tracking of synchronization between the received signal and the spreading code according to the calculated representative value.
In accordance with a second aspect of the present invention, there is provided a receiver including an input unit for producing a despread signal according to a correlation between a received spread spectrum signal and an inputted spreading code, a delay profile detector for producing a delay profile for the despread signal from the input unit, a data symbol demodulator for conducting a rake combining, according to the delay profile produced from the profile detector, for the despread signal from the input unit and producing a decoded signal; a representative value calculating unit for calculating, according to the delay profile produced from the profile detector, a representative value representing spreading code phases of a plurality of paths using a plurality of correlation values and code phases corresponded thereto, and a clock control unit for achieving a control operation to conduct a tracking of synchronization, according to the representative value from the representative value calculating unit, between the received signal inputted to the input unit and the spreading code.
In accordance with a third aspect of the present invention, there is provided a base station and a mobile terminal each including the receiver described above.